The more you read this novel, the more it becomes a memoir - indeed, a few chapters in, you find out that the narrator is called Mrs Küng. She has worked for the same newspapers as the author, lived in the same places. So, what is the difference between Küng's life and her fiction? Just one thing. In the latter she is visited in her adopted Swiss village by an oddly dressed man, invisible to others, who announces himself as François-Marie Arouet - better known as the acerbic satirist and anatomist of human delusions, Voltaire. He keeps her company, dispenses sardonic aperçus and anachronistic advice, and narrates episodes from his life.

While Küng the narrator chats with Voltaire, Küng the author allows him ingenious 18th-century analogies, often from the dissolute, bitchy courts of Europe, for the preoccupations of her own age. He tells her of his long campaigns against intolerance and superstition, and she fits his words to present-day politics. Meanwhile the author's history is funnelled in: her "seduction" of the man who is to be her husband is the more wince-making for being set in parallel with Voltaire's account of the amours of Louis XV and Madame de Pompadour.

Although it's been longlisted for the Orange prize, this is not really a novel. You have to get through a good deal of the author's life to get a little of Voltaire's, though the fragments he is made to tell do sometimes come alive. There is, however, the uneasy sense that the author/narrator feels that her own life gives her some kind of affinity with the outrage-causing penseur, driven from one land to another by the fearless mischief of his own ideas.

I know the feeling. When life lours, it would be good to have a companion from the Age of Enlightenment: David Hume to get you through the tedium vitae, Dr Johnson to help you over writer's block. No doubt it is the fantasy of many a captivated reader. Yet to act it out over 350 pages is weirdly self-regarding. An account of the eccentricities of the Swiss would have been acceptable. A fictionalisation of Voltaire's life would have been fine. But somehow this attempt to blend the two is exasperating.